This invention relates to radio frequency antenna couplers and in particular to very high frequency antenna couplers.
To ensure maximum power transfer between a power source and a load, as is known in the art, the impedance of the transmission line between the power source and the load must be equal to the output impedance Z.sub.0 of the load. The impedance of the transmission line and the load is not a constant impedance and there are many variables which will drastically affect the characteristic impedance of the transmission line and the load. Examples of these variables are the frequency of the signal that is being applied to the transmission line and the load, environmental conditions, loose manufacturing tolerance of the transmission line and the load, and combinations of the above enumerated conditions. In situations where there is a very high frequency signal, such as that generated by a radio transmitter and in particular a very high frequency transmitter; the above enumerated conditions can drastically effect the operation of a transmitter and the transfer of power between the radio frequency source and the load which is normally an antenna. To deal with this situation in the prior art, tuning stubs are used to place in parallel with the transmission line and load a parallel resistance component that matches the load to the output impedance of the radio frequency source. The tuning stub is then adjusted, through its location on the transmission line, to cancel out the residual reactance at the tuning location point. If the stub is connected behind a voltage maximum, the tuning stub is short circuited and will not exceed a quarter-wavelength, .lambda./4, at the operating frequency. The tuning stub must be repositioned and adjusted for each new frequency selected by the radio frequency source and for any change in the environmental conditions.
An impedance matching network which can also be described as a lumped element tuning stub for matching a generator to a load over a wide range of frequencies and impedances was disclosed in U.S. Pat. No. 4,095,198. The network provided a pair of buses having a plurality of capacitors that can be selectively connected to achieve an overall desired value of capacitance, and a second pair of buses across which a plurality of inductors may be selectively connected to achieve an overall desired value of impedance for the network. The network can be controlled so that the capacitance and inductance elements may be arranged with respect to each other, the generator and load in any one of four possible L type configurations. The above described network provides a means for selectively tuning a transmission line to achieve a desired impedance matching tuning stub or filter. However, there was no provisions provided for dynamically matching the impedance to be detected condition.
In a similar type circuit disclosed in U.S. Pat. No. 3,775,707 there was disclosed an improved antenna coupler having reactive elements for translating antenna impedance to a resistive load, and an auto transformer for transforming the resistance load into a resistive level for matching with the resistance of the RF source. The auto transformer included a magnetic core and a coil having a plurality of turns, tap contacts to select turns of the coil and a switch means for connecting one of the taps of the coil to the antenna and on the other side for connecting one of another set of taps to an inductive means. Here again impedance matching was provided but there was no means for determining the desired impedance.
In U.S. Pat. No. 3,906,405 there was disclosed a tunable antenna coupling circuit for applying signals of different frequencies between an antenna and a transmitter-receiver and included a series circuit having a plurality of inductance sections and one or more shunt circuits providing capacitance between the series circuit and a reference potential. Switches are connected across the inductance sections and selectively connect the capacitors in the shunt circuits to control the effective values in the circuit to match the impedance of the antenna at different frequencies and to efficiently appy signals between the antenna and the transmitter-receiver. The switches are controlled by the channel selector and provide for different channel frequencies to be coupled to the switches through a diode matrix for selectively operating the reed switches. The position of the channel selector controls the operation of a preselected number of the switches to provide the desired coupling impedance for each channel. The value of the inductance sections and the value of the capacitance have a binary relation so that by selective connection in the coupling arrangement a wide range of impedance values and capacitance values are obtained in small incremental steps. The above enumerated patent tied the antenna coupler to the frequency of operation but did not provide for the situations where environmental conditions also influence the impedance of the transmission line and antenna in addition to the frequency of operation of the radio.
Another means for ensuring the proper power transfer between a radio transmitter and the antenna was provided for in U.S. Pat. No. 3,366,883 which provided a control device that controlled the flow of power from the radio frequency source to the antenna. Interposed between the radio frequency source and the antenna system was a voltage control attenuator which attenuated the voltage in response to a sense signal that was the summation of the measured forward power and reflected power. The flow of power was thus regulated as a function of the forward power to a given value for matched conditions and is reduced as a function of the amount of mismatch indicated by the presence of the reflected power from the load.